Frameshift mutation in the Tgs1 coding sequence, resulting in an early stop codon. A dicistronic locus encodes both the moi (upstream) and Tgs1 (downstream) open reading frames (ORFs). The lethality of the Tgs1R2 mutation is rescued by a transgene containing only the Tgs1 ORF, and is not rescued by a transgene containing only the moi ORF.